New member with PAS 3

Hello All, I have just joined the forum, and I'm looking for advice regarding a PAS 3 that I recently re-discovered in the attic. I used it back in the late '70s - early '80s as the preamp in a turntable based system based around a Thorens 125, Grace arm, Supex 901, into the PAS3, then a Luxkit power amp, and finally early Rogers ls3/5a speakers. When properly set up, this system sounded fabulous, but then the PAS transformer shorted out, and a few other problems caused the whole system to be sidelined for many years.. Now I want to put it all together again based around a modified Rega planar 3 with a Denon dl110 high output MC. ( I have sourced a replacement trans former for the PAS and still have the LUX and ls3/5a's) I have replaced the two power supply caps in the filament circuit, and will probably replace the quad can as well, but my main question at the moment is would it be possible to just use the phono stage of the PAS to drive the power amp directly from the volume control, bypassing the line stage, eq etc., using it as a minimalist phono only preamp as such. Any advice would be appreciated. Thanks, keego38

You might be able to do it if the input impedance of your amplifier is high enough.

How high is enough? Check me on this, but I believe the output of the phono section directly drives the tape out jack in the original design. How high was the input impedance of tape recorders in those days? If you can research this online by finding the manuals for some old tape decks from, say, the 1960s, it might give you an idea of how high the amplifier’s input impedance would need to be.

On the other hand, if the input impedance of those tape decks was higher than what you'd actually need, then this information might not be of any real help. But, if it turned out to be similar to the input impedance of your amplifier, then maybe it might work.

Then there is also the problem of level control. You'd either need an amplifier with its own gain control or you'd still have to go through the PAS volume pot, which might need to be replaced.

Your cartridge would also need enough output to drive the phono section, as well - this would be even more important since there would be no active line stage after it.

Do you know what the specification is for the input impedance of your amplifier?

I would also question what the implications might be for the line stage card with its input open but power still applied – I’d want to know that this would not stress the two tubes in the line stage, especially if they are older, rarer tubes.

So, there would be a number of considerations. It might not be as easy as just rerouting a couple of wires.

PeterCapo wrote:You might be able to do it if the input impedance of your amplifier is high enough.

How high is enough? Check me on this, but I believe the output of the phono section directly drives the tape out jack in the original design. How high was the input impedance of tape recorders in those days? If you can research this online by finding the manuals for some old tape decks from, say, the 1960s, it might give you an idea of how high the amplifier’s input impedance would need to be.

On the other hand, if the input impedance of those tape decks was higher than what you'd actually need, then this information might not be of any real help. But, if it turned out to be similar to the input impedance of your amplifier, then maybe it might work.

Then there is also the problem of level control. You'd either need an amplifier with its own gain control or you'd still have to go through the PAS volume pot, which might need to be replaced.

Your cartridge would also need enough output to drive the phono section, as well - this would be even more important since there would be no active line stage after it.

Do you know what the specification is for the input impedance of your amplifier?

I would also question what the implications might be for the line stage card with its input open but power still applied – I’d want to know that this would not stress the two tubes in the line stage, especially if they are older, rarer tubes.

So, there would be a number of considerations. It might not be as easy as just rerouting a couple of wires.

- Peter

Hi Peter, I've been thinking about level and indeed impedance matching, and as far as I can make out, typical input impedance for most tape decks is about 20K, with input sensitivity of 175ma. My moving coil cartridge only puts out 2mv so I don't think I would have sufficient gain to drive the power amp sufficiently.. What I have decided to do is bypass the selector/balance/filter/tone controls, and replace the volume control with a good quality ALPS 250k stereo pot, and continue using the line driver stage. When I get everything up and running I may experiment with the original idea, bearing your advice in mind- will post again when I get that far, should be within the next month or so. I'm definitely going to upgrade the power supply though, Bob sent me a link to Tubes4hifi and their supply looks like just the job. Thanks Guys, Paul

Typical of tape decks from which era? Tape recording machines during the time when the PAS was designed probably had vacuum tube input stages, which I would think might have had significantly higher input impedance. 20k Z-in would be too low for either the phono stage or even the line stage to drive without the low frequencies rolling off. The question about the cartridge's output is, of course, not that the cartridge would be driving the amplifier’s input stage. The cartridge's output would be amplified by the PAS phono section before the signal goes anywhere else, whether to the line card or straight to the power amp. But, if the cartridge’s output is on the low side, then the phono stage might not be able to boost it enough, level-wise. Well, the fun is in experimenting, anyway. Good luck!